Journal of the Korean Physical Society, Vol. 46, No. 1, January 2005, pp. 120∼123
Optical and Structural Properties of Self-Assembled ZnO Nanocrystals Tae-Bong Hur, Yoon-Hwae Hwang and Hyung-Kook Kim∗ Department of Physics and Research Center for Dielectric Advanced Matter Physics, Pusan National University, Busan 609-735
Ik Jae Lee Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784 (Received 5 August 2004) Self-assembled ZnO nanocrystals were fabricated by using a RF-magnetron sputtering system. The average diameters of the nanocrystals with different deposition times were roughly 41 nm and 62 nm. The nanocrystals had a preferred orientation to c-axis in the surface-normal direction, but were disordered in the in-plane direction. Photoluminescence measurements of the nanocrystals revealed an intense UV peak at 3.380 eV at 15 K (with a width of 65 meV), which corresponds to the free exciton energy. PACS numbers: 61.46, 61.10, 78.55.C Keywords: ZnO, Photoluminescence, Nanocrystals
disordered system in the in-plane direction. Photoluminescence spectra were measured at temperatures ranging from 15 to 300 K. The free exciton transition energy and peak width at 15 K were 3.380 eV and 65 meV, respectively. As the temperature increased, the free exciton transition energy decreased.
I. INTRODUCTION
Zinc oxide is a II-VI group compound semiconductor with a high exciton binding energy of 60 meV, which provides efficient emission at room temperature. It is an oxide semiconductor with a band gap of 3.36 eV at room temperature and is an attractive material because of its application to optoelectronics [1–3]. Recently, many groups have studied the fabrication of ZnO nanocrystal because unique properties such as UV nano-laser activity have been demonstrated [4, 5]. The reported ZnO nanocrystals were ultraviolet-emitting ZnO nanowires [6], ZnO nanorods by metalorganic vapor deposition [7], ZnO nanotubes by a thermal evaporation method [8] and selective ZnO nanodots formed on a nanopatterned substrate [9]. However, a self-assembled ZnO quantum dot grown on the substrate was not reported. Although the mechanisms of self-assembled growth are poorly understood at present, there is no doubt that this approach opens extensive prospects for application of nanocrystals. The self-assembled quantum system is a promising candidate for the fabrication of quantum devices without any lithography step. In this paper, we report the optical and structural properties of self-assembled ZnO nanocrystals on Pt(111) substrate by using a RF-magnetron-sputtering system. The average diameters of the nanocrystals with different sputtering times were roughly 41 nm and 62 nm. The nanocrystals had a preferred orientation to the c-axis in the surface-normal direction, but the nanocrystals were a ∗ E-mail:
II. EXPERIMENT Self-assembled ZnO nanocrystals were grown on Pt(111)/TiO2 /SiO2 /Si substrates by using the radiofrequency (RF) magnetron-sputtering deposition method in a gas mixture of argon and oxygen. The purity of both gases was 99.999 %. The basic pressure was 1 × 10−5 Torr and the gas pressure during sputtering was 1.7 × 10−2 Torr. The ratio of O2 to Ar was 2.57 to 2. The Zn target was 2.5 cm in diameter and its purity was 99.999 %. The distance between the target and substrate was nearly 10 cm. The RF power was fixed at 50 W. Synchrotron X-ray scattering measurements were performed at the 3C2 beamline of the Pohang Light Source (PLS). The incident X-ray was focused by a toroidal mirror and was monochromated to 1.5402 ˚ A by a double-bounce Si(111) monochromator. A Si(111) analyzer crystal was used to provide a highresolution configuration in reciprocal space. The surface morphology and average diameter of self-assembled ZnO nanocrystals were measured by scanning electron microscopy (SEM : Model S-4200, HITACHI) and were analyzed by image-pro-plus program. The optical properties of the nanocrystals were investigated by
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Optical and Structural Properties of Self-Assembled ZnO Nanocrystals – Tae-Bong Hur et al.
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photoluminescence (PL) with a He-Cd laser (325 nm) (κ-KIMMON) in a wide temperature range (15 – 300 K).
III. RESULTS AND DISCUSSION When a thin film is grown on a substrate, the thin film growth process generally obeys the Stranski-Krastanow model [10]. This model explains the two-step growth process. In the first stage, the well-aligned 2-D layer is completed. In the second stage, the 3-D islands are grown due to a lattice strain by the lattice mismatch between the substrate and the film. Volmer-Weber growth, in which the 3-D islands are directly grown on the substrate without the well-aligned 2-D layer growth [10], is not accomplished. Surprisingly, the self-assembled ZnO nanocrystals grown on Pt(111) substrate seem to follow the Volmer-Weber growth model. Figure 1 shows SEM images of self-assembled ZnO nanocrystals grown on Pt(111) substrate. The average diameters in figures 1(a) and (b) are roughly 41 nm and 62 nm, respectively. The SEM images are shown on a 600-nm scale. The morphology of the nanocrystals was hexagonal and is clearly shown in figure 1(a). The preferred orientation of the nanocrystals was the c-axis of ZnO hexagonal structure in the surface normal direction. The deposited zinc atoms have high kinetic energy because of the relatively high substrate temperature. Adsorbed zinc and oxygen atoms forming discrete nucleations were grown. The growth of discrete nucleations was accomplished according to the Volmer-Weber model of self-assembled ZnO
Fig. 1. SEM images of self-assembled ZnO nanocrystals grown on Pt(111) substrate in different growth times. The average diameters of (a) and (b) were roughly 41 and 62 nm, respectively.
Fig. 2. Size distributions of self-assembled ZnO nanocrystals represented with histograms. The distributions were obtained by using the Image-Pro-Plus program. The average diameters of (a) and (b) are roughly 41 and 62 nm, respectively.
nanocrystals. The areas indicated by arrow A in figure 1(a) seemed to be Pt(111) substrate surface. The nearest neighbor center-to-center distance of the self-assembled ZnO nanocrystals was related to the diffusion length and migration distance of the deposited zinc atoms. A peanut type of ZnO nanocrystals indicated by arrow B was the consequence of a coalescence process by migration [10, 11]. Also, the discrete growth of a second layer of ZnO nanocrystals could be accomplished, as indicated by C in Figure 1(b). Figure 2 shows the size distribution of the selfassembled ZnO nanocrytals, analyzed by the Image-ProPlus program. The histogram of Figure 2(a) shows an asymmetric shape. The average diameters and half widths in Figure 2(a) and (b) were roughly 41 nm (17 nm) and 62 nm (28 nm), respectively. The average diameters of the nanocrystals were controlled by the sputtering times of 20 and 30 min. Figure 3 shows the longitudinal and transverse spectra of the 62-nm-size nanocrystals from the ZnO(0002) Bragg peak by the synchrotron X-ray scattering experiment (PLS). The self-assembled ZnO nanocrystals had the preferred orientation of the ZnO[0002] direction parallel to the Pt surface-normal direction. The c-lattice constant of the nanocrystals was the same as that of bulk crystal. The other peaks such as ZnO(10¯10) or ZnO(10¯11) in the longitudinal scan did not appear. The longitudinal scan at the ZnO(0002) peak in figure 3(a) clearly shows the interference fringes at high q values. This indicates that the nanocrystals have an atomically well-ordered finite-size domain. The intensity of the atomically well-ordered finite-size domain is represented by I ∝ sin2 (Nqc/2)/sin2 (qc/2), where q = 4π sin θ/λ, N is the number of unit cells, and c is a lattice constant [12, 13]. The domain size calculated from the spacing, δq, of the fringes was roughly 305 ˚ A by using d = 2π/δq. The correlation length of the ZnO nanocrystals, calcu-
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Fig. 3. Longitudinal and transverse spectra of the 62-nm size at ZnO(0002) Bragg peak. The spacing between dotted lines in (a) corresponds to the difference in the momentum transfer, δqz .
Figure 4 shows photoluminescence spectra of the selfassembled ZnO nanocrystals of 62 nm size. The inset of Figure 4 shows the PL spectrum at 15 K. This spectrum shows the free exciton transition (3.380 eV) and greenband emission (2.38 eV) [15,16], respectively. The width of the free exciton peak at 15 K was very large (∼ 65 meV) due to the size distribution of the nanocrystals. Also, the width of the free exciton peak at room temperature was roughly 120 meV. The intensity and width of green-band emission were very small. Regarding the broad-band emission located at about 2.76 eV, we speculate that it comes from defects or naturally formed Si nanocrystals in SiO2 [17, 18]. The PL spectrum of the self-assembled ZnO nanocrystals at 15 K had no defectrelated peaks such as bound exciton emission or DAP emission. The free exciton transition of the nanocrystals was studied by measuring the temperature-dependent photoluminescence spectra in the temperature range of 15 – 300 K. As the temperature increased, the free exciton emission peak energy decreased and its width increased. The green-emission peak observed in the inset of figure 4 disappeared at 45 K. The variation of free exciton energy as a function of temperature satisfied Varshni’s formula. The blue-shift of ZnO nanocrystals by quantum-confinement effects was ambiguous. It seems that the apparent quantum-confinement effects of self-assembled ZnO nanocrystals can be expected in the smaller-size nanocrystals.
IV. CONCLUSIONS
Fig. 4. Photoluminescence spectra of the self-assembled ZnO nanocrystals of 62-nm size in a wide temperature range (15 – 300 K). The dotted line is a guide to the eyes. The inset shows the PL spectrum at 15 K.
lated from the width of the main peak (FWHM = 0.0195 ˚ A−1 ) in figure 3(b), was roughly 320 ˚ A in the surfacenormal direction and is consistent with the value calculated from the spacing of the fringes. The FWHM of 2.8348◦ in the transverse scan as shown in Figure 3(c) indicates good nanocrystal quality. In the transverse scan at the ZnO(0002) peak, a specular component from the well-aligned 2-D layer was not observed, as shown in figure 2(b) [13,14]. The intensity of the in-plane φ-scan at the ZnO(10¯11) peak was constant (data are not shown here). This result indicates that the structure distribution of the self-assembled ZnO nanocrystals was powderlike in the in-plane direction.
In conclusion, we successfully fabricated selfassembled ZnO nanocrystals on Pt(111) substrate. The nanocrystals had a preferred orientation to the c-axis in the surface-normal direction, but the in-plane ordering of nanocrystals was random according to units of single nanocrystals. The temperature-dependent photolumines cence spectra show the free exciton transition.
ACKNOWLEDGMENTS This work was supported by the ministry of Science and Technology through Nanoscopia Center of Excellence at Hanyang University.
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